Hematopoietic precursors are vital for continuous replenishment of all myeloid and lymphoid cells. The objective of this proposal is to determine how diet restriction (DR) alters aging of hematopoietic stem cells (HSCs) and HSCs-Expanded (HSCs-EXP), a newly-defined category of stem cells responsible for expansion of HSC function in irradiated carriers. This work explores the exciting possibility that DR retards aging by altering stem cells. The long term goal is to improve clinical treatment of stem cell aging. The following hypotheses are tested: Aim 1: DR alters aging of stem cells in BALB and D2 mice. This hypothesis predicts 1) that, like BALB mice, D2 mice treated with DR will show increased HSC function with age, and thus show greater myeloid and lymphoid repopulating abilities than young controls; and 2) that proportions of donor derived differentiated cells will remain stable in DR-treated mice after 4 months, showing HSC clonal stability. Aim 2: DR alters the function of B6 stem cells, by either l) adding to the advantage of old over young B6 marrow cells, or 2) making old similar to young controls, suggesting that DR prevents an age-related impairment ' for which old B6 mice normally over-compensate by enhancing HSC function. Aim 3: DR alters aging of BALB stem cells by increasing proliferation, homing, or both in mice that are irradiated or shielded except for their tibias; HSCs from irradiated tibias and shielded tibias are compared. Aim 4: DR's specific endocrine effects retard stem cell aging. This hypothesis predicts that loss of HSC function with age can be reversed in old non-DR mice with specific endocrine mutations, which, like DR, increase maximum life spans. Effects of diminished circulating IGF-1 will be tested in "little" mutants, while combined endocrine effects that partly mimic DR will be tested in Snell dwarf mutants. Effects of elevated circulating corticosterone, which also occurs with DR, will be tested in normal mice and in both mutants. For all aims, both HSCs and HSCs-EXP will be assayed using competitive repopulation and enumerated using limiting dilution. In addition, bone marrow subsets containing HSCs will be quantified by FACS, using HSC markers. The assays are feasible due to the genetic resources and animal expertise at The Jackson Laboratory, as well as the applicants' extensive experience in evaluating HSC functions.